The present invention relates generally to the preparation of animal feed and more particularly to the preparation of mash for subsequent formation into animal feed pellets.
Feed for chickens, swine, and other animals raised for human consumption traditionally is prepared from a mash. The mash is prepared according to a recipe and may include a mixture of many ingredients including, for example, cereal grains, plant and animal proteins, fats, roughage, liquids, and other products in predetermined proportions. The mash usually is delivered from a holding bin to a conditioner in which moisture and heat are applied to the mash to cook and condition it. From the conditioner, the conditioned mash may be delivered to a pellet mill, where it is extruded through dies into feed pellets, which subsequently are cooled and dried to form the final feed product for consumption by animals.
Traditional pellet mills for extruding mash into feed pellets may include foraminous annular dies with internal rollers that force the mash though openings in the dies to extrude it into pellets. Pellet mills generally are driven by electric motors that may vary anywhere from 25 to over 600 horsepower. Extruding the mash through a die is a complex Theological process of deformation and flow conditioned upon stress, strain, and time. The load on the die and thus the efficiency of the pellet mill and the quality of the pellets produced primarily is dependent upon the rate at which mash is fed to the pellet mill and the composition of the mash, its moisture and temperature content, the degree to which it is gelatinized or plasticized and the degree of its deaeration.
Improving the gelatinization or plasticization of the mash advantageously results in decreased horsepower requirements, better pellet quality, and increased feed production efficiency. Unfortunately, many conventional feed mills lack the high intensity, high temperature conditioning required to bring about gelatinization levels necessary to realize these advantages. Further, the addition of steam of more than abouth 3% in conventional systems causes problems for the pellet extrusion process and results in an increase in temperature of only about 50 degrees C, which generally is too short lived to create the desired conditioning effects. Additionally, in conventional devices, high fat content acts as a barrier to moisture thereby obstructing its diffusion into the mash and resulting in poor pellet quality.
A variety of attempts have been made to improve the conditioning of the mash prior to its extrusion into pellets in a pellet mill. For example, expanders have been developed for treating the mash after it has been conditioned in a conditioning chamber but before it is fed to a pellet mill. In general, an expander is a mechanical device that subjects the mash to extreme mechanical pressure, friction, and shearing forces. These extreme conditions heat the mash through conversion of mechanical energy and the resulting friction to heat and tends to rupture the starch molecules and cause the feed particles to aggregate into clumps. As a result of these and other effects, gelatinization and plasticization of the mash are improved, which improves the efficiency of extrusion into pellets, enhances the pellet quality, enhances protein digestability within the final feed product, and results in faster glucose influx in the gut after feed consumption. It also is believed that subjecting the mash to the drastic pressure drop upon leaving the expander results in rupture of the cell walls and consequent killing of undesirable micro-organisms such as salmonella within the mash. Once the mash exits an expander, it is cooled prior to entering the pellet mill, where it is extruded into pellets.
While expanders certainly have improved the conditioning of mash prior to extrusion into feed pellets, they nevertheless have certain inherent problems and shortcomings. Primary among these is the amount of costly energy in the form of electricity required by the electric motors that power such expanders. Because of the inherent inefficiencies in converting mechanical energy into the heat and stress required to improve the quality of the mash, motors in the several hundred horsepower range are required and these motors draw large electrical currents and are expensive to operate. For example, typical throughputs of from about 1.5 to about 30 tons of feed produced per hour can require from about 75 to about 315 kW per hour of electricity just to operate the expander. In many instances, these increased costs are prohibitive. Further, the high stresses and mechanical action of expanders means that they are subject to frequent maintenance and repair, which results in production down time and even higher operating costs.
At least one alternative to mechanical expanders has been developed. U. S. Pat. No. 5,744,186 of Harrison discloses a process for preparing animal food pellets that is claimed to obtain the advantages of expanders without their high costs of operation. The Harrison process contemplates a superatmospheric mash conditioning chamber that essentially is a pressure cooker within which the mash is subjected to high pressures and high temperatures of about 30 psi and about 260 degrees F from the time it enters the conditioning chamber until it leaves the die as pellets. An expander is not used in the Harrison process and continuous high pressure is maintained between the conditioning and extrusion steps. Harrison contends that this pressure cooker-type conditioning results, inter alia, in better pellet characteristics than expanders, increased geletanization and plasticization of the mash for improved extrusion efficiency, increased digestibility, increased nutritional value of the feed pellets, increased amino acid content, and advantageous inactivation of undesirable micro-organisms such as salmonella, mycotixins, karnal bunt, botulism, listeria, E. coli, and others.
While the superatmospheric conditioning chamber of Harrison may have certain advantages over expanders, it nevertheless also carries its own problems and disadvantages, some of which are acknowledged by Harrison itself. For instance, because of the requirement to maintain high pressures and temperatures throughout the conditioning process, Harrison discloses that the chamber as well as the die and roller apparatus of the pellet mill must be specially designed to withstand continuous high temperatures and pressures. Further, the maintenance of high pressure requires that the entire production line from the input hopper to the pellet mill itself be sealed in some fashion. These seals are provided by a specialized compression plug seal at the inlet hopper, which requires additional energy, and other types of seals throughout the mechanism. Accordingly, while Harrison may be an advance over traditional expanders, it is far from a completely satisfactory solution to the problem.
A need therefore exists for an improved method and apparatus for preparing mash for the production of animal feed pellets that successfully addresses the problems and shortcomings of the prior art. Such a method and apparatus should result in high quality animal feed pellets without the high energy consumption of expanders or the problems attendant the pressure cooker approach of Harrison. It is to the provision of such a method and apparatus that the present invention is primarily directed.
Briefly described, the present invention, in a preferred embodiment thereof, comprises a method of preparing mash for extrusion into animal feed pellets. The method, in one embodiment thereof, includes the steps of introducing the mash into a pre-conditioning chamber and heating the mash to a first temperature within the pre-conditioning chamber by injecting steam at a temperature of about 220 degrees F into the pre-conditioning chamber with the mash. The mash is pre-conditioned and heated by the steam to a first temperature of about 160 degrees F as it is conveyed by a mixing screw assembly along the pre-conditioning chamber from the upstream end to the downstream end thereof. At this temperature, the mash is pre-conditioned and the cooking of starches within the mash is initiated.
From the downstream end of the pre-conditioning chamber, the mash is transferred to the upstream end of a separate conditioning chamber, where it continues to be mixed and is conveyed by a mixing screw assembly to the downstream end of the conditioning chamber. Superheated steam, which has been heated in a superheater to a temperature between about 480 and 520 degrees F, is injected at the upstream end of the conditioning chamber and mixes with the mash as it moves along the length thereof. The superheated steam raises the temperature of the pre-conditioned mash to a second temperature of about 200 to 215 degrees F as it mixes with and moves along the conditioning chamber with the mash. This is a sufficient temperature to continue the cooking of the starches in the mash, expand and gelatinize the mash, kill potentially toxic micro-organisms such as salmonella and E. coli, and to deactivate most other potential toxins in the mash.
The total residence time of the mash in the pre-conditioning chamber and the conditioning chamber preferably is from about one minute to about one minute thirty seconds and most preferably about one minute fifteen seconds. From the conditioning chamber, the now completely conditioned mash is transferred to a conventional pellet extrusion mill, where it is extruded into pellets in the traditional way prior to being cooled and dried for transport and use.
The method of the invention obtains results that are equal or superior to those obtained with conventional expanders, but with significantly less required energy input and substantially lower maintenance requirements. Further, both the pre-conditioning chamber and the conditioning chamber operate at atmospheric pressure rather than the elevated pressures that must be maintained in pressure cooker type feed mill systems. Thus, the requirement for seals throughout the system to maintain the high pressures and the requirement that all of the internal components of the system be designed for pressurized operation are eliminated. Further, since the temperature of the steam injected into the pre-conditioning chamber and of the superheated steam injected into the conditioning chamber can be precisely controlled, the final temperatures reached by the mash in these chambers, which can be monitored, also can be controlled precisely. As a result, the quality of the finished feed pellets can be kept consistently high throughout long operating cycles by continuously monitoring and adjusting the feed rates, temperatures, and other parameters of the process.
The apparatus for carrying out the method of the invention comprises an elongated pre-conditioning chamber with an upstream end and a downstream end and an elongated conditioning chamber, also with an upstream end and a downstream end. An inlet chute is provided at the upstream end of the pre-conditioning chamber for introducing raw mash into the chamber and a rotating mixing screw assembly is disposed in the pre-conditioning chamber to mix and convey the mash toward the downstream end of the chamber. A steam injection manifold injects steam into the upstream end of the pre-conditioning chamber for mixing with, pre-heating, and pre-conditioning the mash in the pre-conditioning chamber. A chute is provided for transferring pre-conditioned mash from the downstream end of the pre-conditioning chamber into the upstream end of the conditioning chamber. Superheated steam from a superheater is injected through a steam injection manifold at the upstream end of the conditioning chamber and the superheated steam is mixed and conveyed along the conditioning chamber with the mash by a mixing screw assembly within the conditioning chamber. In this way, the mash is cooked, expanded, and gelatinized as described above. An outlet chute is provided at the downstream end of the conditioning chamber for transferring the mash to a conventional pellet extrusion mill, where it is extruded into feed pellets.
In an alternative embodiment of the invention, the superheated steam at a relatively higher temperature of between about 480 and 520 degrees F, is injected into the pre-conditioning chamber and steam at a relatively lower temperature of about 220 degrees F. is injected into the conditioning chamber. Thus, in this embodiment, the higher temperature superheated steam is injected into the pre-conditioning chamber rather than the conditioning chamber. The resultant feed pellets exhibit qualities that are essentially the same as with the first embodiment, but the mash is initially heated to a higher temperature in the pre-conditioning chamber by the superheated steam. Therefore, the scope of the invention includes injecting superheated steam either into the pre-conditioning chamber or into the conditioning chamber, or both, depending upon application specific goals and parameters.
Thus, a method and apparatus for preparing animal feed from mash is now provided that successfully addresses the problems and shortcomings of the prior art. Inefficient expanders are eliminated as is the requirement for maintaining high pressures within the system during processing. The resulting feed pellet product is of consistently high and highly controllable quality and is produced with substantially less total energy per ton than is possible in the prior art. These and other features, objects, and advantages of the invention will become more apparent upon review of the detailed description set forth below when taken in conjunction with the accompanying drawing figure, which is briefly described as follows.